Recently, portable electronic apparatuses such as mobile phones, notebook computers, and personal digital assistants (PDAs) have gained widespread used. These portable electronic apparatuses are commonly equipped with one or more rechargeable batteries. In order to ensure portability of such portable electronic apparatuses, a separate battery charger is required for charging the batteries of the portable electronic apparatus.
Referring to FIG. 3, a charging circuit 100 of a typical battery charger is shown. The charging circuit 100 includes a power supply unit 110, a load circuit 120, a transistor 131, an amplifier comparator 132, a feedback resistor 1321, two voltage-dividing resistors 1322, 1323, and a sampling resistor 140. The power supply unit 110 includes a power output terminal 111 and a grounding terminal 112. The transistor 131 includes a gate electrode 1311, a source electrode 1312, and a drain electrode 1313.
One terminal of the sampling resistor 140 is connected to the grounding terminal 112 of the power supply unit 110. The other terminal of the sampling resistor 140, namely a current sampling terminal, is connected to the power output terminal 111 of the power supply unit 110 via the load circuit 120, the drain electrode 1313 of the transistor 131, and the source electrode 1312 of the transistor 131. The drain electrode 1313 of the transistor 131 is connected to the grounding terminal 112 of the power supply unit 110 via the two voltage-dividing resistors 1322, 1323. A node between the two voltage-dividing resistors 1322, 1323 is connected to a negative input terminal of the amplifier comparator 132. The current sampling terminal of the sampling resistor 140 is connected to a positive input terminal of the amplifier comparator 132. An output terminal of the amplifier comparator 132 is connected to the gate electrode 1311 of the transistor 131. Two terminals of the feedback resistor 1321 are connected with the output terminal and the negative input terminal of the amplifier comparator 132, respectively. That is, the feedback resistor 1321 is connected with the amplifier comparator 132 in parallel.
The power supply unit 110 transforms alternating current to direct current having a lower voltage, and charges the load circuit 120 through the power output terminal 111. The amplifier comparator 132 switches the transistor 131 to control a charging current and a charging voltage of the charging circuit 100. When the charging circuit 100 works normally, the transistor 131 is in an on-state, and the charging current travels through the transistor 131. However, because the transistor 131 has high power, a large heat sink is needed. Thus, a volume of space required by the charging circuit 100 is large.
Therefore, a new charging circuit that can overcome the above-described problems is desired.